Method of protecting electrostatic spray nozzles from fouling

ABSTRACT

A method of protection of electrostatic projection nozzles against fouling due to the rebounding of a small proportion of the projected ionized particles from the object to be coated with a liquid-coating product comprises essentially the provision of an auxiliary liquid product in addition to the main liquid coating product, the auxiliary liquid being electrically charged to a high voltage of the same sign as the charge of the liquid coating product, and being projected simultaneously with the liquid-coating product towards the front of the projection nozzle and from at least one point external to the central portion of said nozzle so as to oppose and annul the opposite charge on the rebounding particles. The nozzle is provided on its periphery with a plurality of needles formed by fine, hollow tubes with bevelled free extremities which are charged to the electric potential of the same sign as that of the projection nozzle.

Lacchia Oct. 9, 1973 METHOD OF PROTECTING ELECTROSTATIC SPRAY NOZZLES FROM FOULING Inventor: Adrien Lacchia, Meylan, France Assignec: Air Industrie, Courbevoie, France Filed: June 26, 1972 Appl. No.: 266,398

US. Cl. 239/3, 239/15, 239/424 Int. Cl. B05b 5/00 Field of Search 239/3, 8, 15, 290, 239/423, 424

References Cited UNITED STATES PATENTS 12/1971 Fclici et al, 239/15 Primary Examiner-M. Henson Wood, Jr. Assistant Examiner-Michael Mar Attorney-Joseph F. Brisebois et al.

[57] ABSTRACT A method of protection of electrostatic projection nozzles against fouling due to the rebounding of a small proportion of the projected ionized particles from the object to be coated with a liquid-coating product comprises essentially the provision of an auxiliary liquid product in addition to the main liquid coating product, the auxiliary liquid being electrically charged to a high voltage of the same sign as the charge of the liquid coating product, and being projected simultaneously with the liquid-coating product towards the front of the projection nozzle and from at least one point external to the central portion of said nozzle so as to oppose and annul the opposite charge on the rebounding particles. The nozzle is provided on its periphery with a plurality of needles formed by fine, hollow tubes with bevelled free extremities which are charged to the electric potential of the same sign as that of the projection nozzle.

Patented Oct. 9, 1973 Fig A Fig 2 METHOD OF PROTECTING ELECTROSTATIC SPRAY NOZZLES FROM FOULING The present invention relates to a method and an apparatus intended to prevent fouling of projection nozzles of electrostatic projection apparatus for liquid coating products.

Electrostatic projection nozzles are generally brought up to a high electric potential and are supplied with the coating product, generally together with spraying air. These nozzles may be mounted on all types of installations, both fixed and manual.

The particles of coating product are charged, in a manner known per se, at the level of the projection nozzle, and are then projected in the direction of the object to be coated, in the electric field created between the projection apparatus and the said object which is generally earthed.

Experience has shown that a very small portion of these particles, of the order of 0.3 percent, does not become fixed on the object but leaves its initial charge on the object and rebounds in the direction of the projection nozzle, with a charge acquired by influence on the object having a sign opposite to that of the initial charge. v

These rebound or return" particles are precipitated on to the projection nozzle on which they become fixed, and in the long run cause its partial or even total obstruction in certain cases. In the case of projection of enamels, this phenomenon is particularly troublesome, as the time of correct operation of a nozzle between two cleaning operations is only of the order of a few minutes.

In order to remedy this troublesome phenomenon, it has already been tried to provide the periphery of the nozzles with ionizing points brought up to the hightension voltage. These points have the purpose of creating, on the one hand an electric wind of opposite direction to that of the trajectory of the return particles, and on the other hand to emit ions having a charge opposite to that of the return particles. These ions collide with the return particles and gradually annul their charge, and then give them a charge of the same sign as the particles emitted by the projection nozzle. The

return particles then move away in the direction of the object to be coated before having reached the nozzle.

In reality, experience has shown that a large number of particles which are not repelled byv the electric wind, and the charge on which has not had the time to be annulled and changed in sign by the ions emitted by the points, come into impact with the nozzle and the said points, on which they adhere strongly, in particular in the case of paints, varnishes, enamels and adhesives. A

ball of product then forms on the points which for that In the case of automatic industrial installations comprising a large number of nozzles mounted on fixed projectors or on robots which may work on a programme, it will be appreciated that it is inadmissible to have to stop the installation so frequently in order to clean the assembly of nozzles.

Similarly, in the case of a hand gun, this will be used with greater efficiency if frequent and difficult cleaning operations are avoided.

The present invention makes it possible to avoid these disadvantages and to ensure the correct operation of an installation for the electrostatic projection of a product for at least several hours under the most unfavorable conditions.

The invention consists of a method according to which there is projected, from a point external to the central portion of the projection nozzle, in a direction opposite to that of the return particles, an auxiliary jet of a charged liquid product. The invention also relates to an apparatus for carrying this method into effect, characterized in that the periphery of a projection nozzle is provided with hollow and fine needles, the extremity of which may be bevelled if so required, of the kind employed as medical needles, these needles being brought up to the high-tension voltage, and in that an auxiliary chosen liquid product is projected through the said needles in a continuous or intermittent manner.

In this way:

the return particles are repelled mechanically by means of the jet of auxiliary product, after having trapped these particles in the vicinity of the needles by means of the high electric field created by the said needles;

the bevelled extremities of the needles are continuously cleaned;

there is created a cloud of charged heavy particles emitted by the said needles, which annul and even change the sign of the charge of the return drops much more effectively than the ions known in the prior art.

The invention will be better understood by means of the following description, given by way of example and with reference to the accompanying drawings, in

which:

FIG. 1 shows diagrammatically a view in longitudinal cross-section of an electrostatic projection nozzle for a liquid product according to the invention;

FIG. 2 shows diagrammatically a view in transverse section of the same projection nozzle;

FlG.3 shows diagrammatically a front view in partial cross-section of the same projection nozzle, showing the supply system for the auxiliary product;

FIGA shows diagrammatically a view looking on the top of the nozzle according to the invention.

As shown, the electrostatic projection nozzle according to the invention comprises anozzle body 1, a diffuser 2, an insulating cap 3 and an external nut 4. In a conventional manner, the liquid coating product is introduced at 5 into the diffuser 2 and passes out through the product outlet 6. The spraying air is introduced into the nozzle through the conduits 7 and then, after passing into the vortex chamber, passes out as a turbulent jet of liquid spray through the annular orifice 8. The parts 1 and 2 are of metal and are brought up to a high voltage, an electric field being thus created between the nozzle and the object.

lnvaccordance with the invention, there is added to these conventional portions of the nozzle a metal memher 4 replacing the usual external nut and comprising as shown fine hollow needles 9 with a bevelled extremity of the medical needle" type, located on the outer periphery of the nozzle 8 and directed towards the object to be coated. The part 4 further comprises chambers 10 and 11 for the formation of an auxiliary airproduct mixture, together with an inlet for a device permitting the ejection into the chamber 10 of a mixture of gases, generally of air and finely pulverized auxiliary product. The member 4 and in consequence the needles 9 are brought-up to a high voltage.

The operation of the device is as follows:

in operation, the liquid coating product is atomized, charged electrically and projected in the direction of the object, at the level of the openings 6 and 8. Through the element 13 there is brought into the chamber 10 a mixture of air and an auxiliary liquid compatible with the coating liquid, such as water in the case of enamel, or a solvent in the case of paint.

To this end, the inlet element 13 comprises a central intake conduit 14 for the auxiliary liquid, and an annular intake conduit 15 for compressed air, surrounding the conduit 14, together with a convergent-divergent system which enables the mixture to be effected. The said mixture of air and auxiliary liquid carried out in the element 13 is brought tangentially into the chamber 10 in which it thereby acquires a slight whirling movement which permits a first improvement in the uniformity of the auxiliary air-liquid mixture.

The chamber 10 communicates with a second chamber 11 by conduits 16 formed tangentially in the same direction as the element 13, in order to obtain in the said chamber 11 a perfect vortex effect which ensures good uniformity of the auxiliary air-liquid mixture at the base of each needle 9.

This mixture is ejected through the hollow needles 9 with bevelled extremities. The drops of auxiliary product are charged electrically by the sharpened extremities of the needles 9 brought up to a high tension, and thus form a cloud of electrified liquid particles surrounding the main jet from the nozzle and tending to oppose, following the process previously explained, the return after rebounding on the object, 1 of particles charged with the opposite sign to the particles emitted by the nozzle and the needles. On the other hand, as has been previously explained, the humidity of the auxiliary air-liquid mixture enables a continuous cleaning of the needles 9 to be ensured.

The invention is obviously not limited to the arrangement described, and it is for example possible to imagine the supply of the needles 9 with an auxiliary liquid unmixed with a gas. Furthermore, it is also not essential that the projection device for the charged auxiliary product according to the invention, should form part of the body of the nozzle, and an absolutely independent construction of this latter may perfectly well be employed.

The invention may be utilized in all electrostatic projection installations for liquid products, used in particular in the enamelling and painting industry for metallic or ceramic parts, and also in the paper-making industry, in particular for the glueing and surface treatment of paper.

What I claim is:

l. A method of protection against fouling of the nozzles of electrostatic projection apparatus for liquidcoating products, said method comprising essentially the steps of: providing an auxiliary liquid product in addition to said liquid-coating product; projecting said auxiliary liquid simultaneously with said liquid-coating product; electrically charging said auxiliary liquid product to a high voltage; and projecting said auxiliary product towards the front of the projection nozzle and from at least one location external to the central portion of said projection nozzle.

2. A method as claimed in claim 1, in which said auxiliary liquid product is charged electrically with a charge of the same sign as that to which said liquidcoating product is charged.

3. A method as claimed in claim 1, in which said auxiliary liquid product is a product compatible with the main lqiudi-coating product projected by said nozzle.

4. A method as claimed in claim 1, in which said auxiliary product is a mixture of gas and liquid.

5. A method as claimed in claim I, in which said auxiliary product is a liquid unmixed with any gas.

6. An electrostatic projection nozzle for a liquidcoating product adapted to carry into effect the method as claimed in claim 1, in which said nozzle is provided, on its periperhy, with a plurality of needles constituted by fine and hollow tubes, the extremities of said needles being brought up to an electric potential of the same sign as the electrical high-tension voltage of said projection nozzle.

7. An electrostatic projection nozzle as claimed in claim 6, in which the free extremities of said hollow needles are bevelled.

8. An electrostatic projection nozzle as claimed in claim 6, said nozzle comprising a device for the formation and the injection of an auxiliary gas-liquid mixture into said needles.

9. An electrostatic projection nozzle as claimed in claim 8, in which said injection device comprises a vortex-forming system.

10. An electrostatic projection nozzle as claimed in claim 8, in which said formation device comprises a convergent-divergent system- 

1. A method of protection against fouling of the nozzles of electrostatic projection apparatus for liquid-coating products, said method comprising essentially the steps of: providing an auxiliary liquid product in addition to said liquid-coating product; projecting said auxiliary liquid simultaneously with said liquid-coating product; electrically charging said auxiliary liquid product to a high voltage; and projecting said auxiliary product towards the front of the projection nozzle and from at least one location external to the central portion of said projection nozzle.
 2. A method as claimed in claim 1, in which said auxiliary liquid product is charged electrically with a charge of the same sign as that to which said liquid-coating product is charged.
 3. A method as claimed in claim 1, in which said auxiliary liquid product is a product compatible with the main lqiudi-coating product projected by said nozzle.
 4. A method as claimed in claim 1, in which said auxiliary product is a mixture of gas and liquid.
 5. A method as claimed in claim 1, in which said auxiliary product is a liquid unmixed with any gas.
 6. An electrostatic projection nozzle for a liquid-coating product adapted to carry into effect the method as claimed in claim 1, in which said nozzle is provided, on its periphery, with a plurality of needles constituted by fine and hollow tubes, the extremities of said needles being brought up to an electric potential of the same sign as the electrical high-tension voltage of said projection nozzle.
 7. An electrostatic projection nozzle as claimed in claim 6, in which the free extremities of said hollow needles are bevelled.
 8. An electrostatic projection nozzle as claimed in claim 6, said nozzle comprising a device for the formation and the injection of an auxiliary gas-liquid mixture into said needles.
 9. An electrostatic projection nozzle as claimed in claim 8, in which said injection device comprises a vortex-forming system.
 10. An electrostatic projection nozzle as claimed in claim 8, in which said formation device comprises a convergent-divergent system. 